Focus control method for photolithography

ABSTRACT

A method comprises providing a semiconductor substrate having at least one layer of a material over the substrate. A sound is applied to the substrate, such that a sound wave is reflected by a top surface of the layer of material The sound wave is detected using a sensor. A topography of the top surface is determined based on the detected sound wave. The determined topography is used to control an immersion lithography process.

FIELD

This disclosure relates to semiconductor fabrication tools and methodsof using the tools.

BACKGROUND

In semiconductor fabrication using single and dual damascene methods, aseries of interconnect layers are formed by depositing an inter-metaldielectric (IMD) material, forming a trench in the IMD layer,overfilling the trench with copper (to form a conductive trace), andplanarizing the substrate. Chemical mechanical polishing (CMP) iscommonly used for planarization, to remove all the copper above thesurface of the IMD layer.

Uneven topography can reduce yield and affect device performance. TheCMP process is intended to achieve a flat topography to improve yield.Nevertheless, during CMP, copper and the adjacent IMD material areremoved from the wafer at different rates, creating non-uniformtopography. Line density is known to affect the removal rates of the IMDand copper materials. Generally, the topography impact is greater in adense pattern region than in a low density (“iso”) region. Dishing anderosion are the two most costly topography issues that arise with copperCMP. Dishing occurs when the copper recedes below or protrudes above thelevel of the adjacent dielectric. Dishing is often observed as aconcavity extending across several lines. Erosion is a localizedthinning of the dielectric between two adjacent lines.

If the CMP process leaves an uneven topography, then subsequentprocessing of the substrate may be affected adversely.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a focus problem in the presence of copperdishing.

FIG. 2 is a schematic diagram of an immersion lithography apparatususing SONAR topography measurement to control lens focus.

FIG. 3 is a diagram showing the use of the SONAR system of FIG. 2.

FIG. 4 is a diagram showing details of the immersion lithographyapparatus.

FIGS. 5 and 6 show a wafer having a plurality of integrated circuitdies, each having at least one field with individual thicknessmeasurements.

FIG. 7 is a flow chart of a method for topography measurement andequipment control.

DETAILED DESCRIPTION

This description of the exemplary embodiments is intended to be read inconnection with the accompanying drawings, which are to be consideredpart of the entire written description. In the description, relativeterms such as “lower,” “upper,” “horizontal,” “vertical,”, “above,”“below,” “up,” “down,” “top” and “bottom” as well as derivative thereof(e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should beconstrued to refer to the orientation as then described or as shown inthe drawing under discussion. These relative terms are for convenienceof description and do not require that the apparatus be constructed oroperated in a particular orientation. Terms concerning attachments,coupling and the like, such as “connected” and “interconnected,” referto a relationship wherein structures are secured or attached to oneanother either directly or indirectly through intervening structures, aswell as both movable or rigid attachments or relationships, unlessexpressly described otherwise.

FIG. 1 is a diagram of a focus problem raised by the presence of copperdishing, erosion, and other wafer surface topography abnormalities.

A lithography tool 100 is shown, for selectively exposing a photoresistmaterial. Details of the apparatus are omitted for clarity. A lightsource 102 provides collimated light for exposing the photoresist. Thelight source may be, for example, a solid state 193 nm YAG laser, otherultraviolet laser, or other laser suitable for immersion lithography.

A substrate 100 may be a wafer on which multiple integrated circuits(IC) are to be formed. The substrate 110 can be, for example, a siliconsubstrate, a III-V compound substrate, a glass substrate, or any othersubstrate suitable for IC fabrication. The substrate has a plurality ofactive devices (not shown), above which an interconnect structure 115 isformed.

The interconnect structure includes a plurality of inter-metaldielectric (IMD) layers 120, 130. Although only two layers 120, 130 areshown, the substrate 110 may have any number of IMD layers. For example,configurations of eight to 15 IMD layers are common. The dielectriclayers can be formed of a material such as, an undoped silicate glass(USG), a boron doped silicate glass (BSG), a phosphorous doped silicateglass (PSG), a boron phosphorous doped silicate glass (BPSG), a siliconnitride, a silicon oxy-nitride, a fluorine doped silicate glass (FSG), alow-k dielectric, and extreme low-K (ELK) or a combination thereof.

The dielectric layers 120, 130 have a plurality of conductive patterns122, 124 132, 134 and vias (not shown), which may be copper lines andvias formed by a single or dual damascene process. The substrate mayhave any number of dense regions 112, in which a large fraction of thelocal substrate area is occupied by the conductive material, and ISOregions 114, in which a large fraction of the local substrate area isdielectric material, not occupied by the conductive material.

The substrate 110 may have varying topography due in part to the dishingand erosion. As used herein, the term “topography” refers to the localfeatures and height variations of the surface of the wafer, both withinand between fields, where each IC die on the substrate may have one ormore fields patterned on it by the photolithography process. Thus, thetopography of the substrate 110 includes (intra-field) variations withinan individual field, which are not considered in determining theuniformity of the wafer thickness (i.e., the measure of inter-fieldthickness variation). For example, in FIG. 5, a wafer 110 has aplurality of IC's 600, each patterned by exposing a field. Theuniformity measures inter-field variations between the ICs 600. In FIG.6, the topography of an individual die is represented by various tones(shades of gray). The thickness of the substrate varies within theindividual die, for example at various points 601-609.

Referring again to FIG. 1, the focal length F of the lens 102 is setsuch that the light 144 reaching the plane 140 above the ISO regions 114of the substrate 110 is focused, but the light 142 reaching the sameplane above the dense regions 112 is out of focus. In particular, thesurface of portion 143 of the substrate 110 receives out-of-focus light.As a result, the varying topography can result in varying sharpness whena photoresist over the substrate is patterned.

The inventors have determined that the focusing problem can be addressedby measuring the topography of the photoresist layer (leveling) afterthe CMP step, and feeding the topography information forward to thephotolithography system, for within-field correction of the focus and/orenergy level of the laser. The focus and energy can be adjustedseparately for each of multiple locations within the same field.

In a photolithography system having an air medium between the lens andthe substrate, an optical system may be used to measure the topography.However, when a light is directed at the surface of the photoresistlayer, a portion of the light may be transmitted through the photoresistand reflected by the underlying layer, causing interference with thelight that is reflected directly from the top surface of thephotoresist. Also, if an immersion lithography system is used, a liquidmedium is provided between the lens and substrate. The liquid medium maybe, for example, ultra pure water (UPW) or other immersion medium thathas a suitable refractive index and does not react with the substratematerials. Thus, a method of measuring the topography is used, which issuitable for measurement while the surface of the substrate is under aliquid medium.

FIGS. 2 and 3 show an immersion photolithography tool having a chamber200 configured for selectively exposing a portion of a photoresist layer150 above a substrate 110. The tool has a light source 102 for exposinga field of the photoresist layer 150.

A sound source 208 is positioned to emit a sound 213 into the chamber200, such that the sound is reflected from a top surface of thephotoresist layer 150.

A sound detector 210 is positioned to receive the reflected sound 215.The sound source 208 and sound detector 210 are both positioned foroperating within the chamber 200 for in situ measurement of the localthickness within the same chamber in which the photoresist 150 isexposed. Thus, the substrate 110 can be moved within the chamber 200directly from the leveling station to the immersion lithography station.Because the substrate 110 is not removed from its carrier beforeproceeding to the immersion lithography tool, the substrate is stillwell registered with its carrier when it reaches the lithographystation.

The sound source 208 and sound detector 210 may both be piezoelectrictransducers. A piezoelectric transducer is capable of converting anelectric voltage applied to it into a mechanical strain resulting in asound wave. A piezoelectric transducer is also capable of converting asound wave (strain) into an electrical voltage (or electric charge). Asan example, transducer 208 may include a piezoelectric thin film thatacts as a driver. When a voltage pulse is applied to the transducer 208,the voltage is converted by the piezoelectric thin film into the soundwave 213. The sound detector 210 can similarly include a piezoelectricthin film that acts as a detector for converting the reflected soundwave 215 back into a voltage. Suitable piezoelectric thin films include,but are not limited to, polyvinylidene difluoride (PVF₂) andtetrafluoroethylene.

A processor 206 includes a module for generating, receiving, andanalyzing the sound waves. Processor 206 may include one or moreprogrammed processors, and may also include one or more applicationspecific circuits (not shown). In one embodiment, for generating thesound waves 213 a pulse generator (not shown) directs a signal through asignal amplifier (not shown) to the transducer 208.

In a similar manner the sound detector 210 may have a piezoelectric filmfor detecting the reflected sound waves from the photoresist andconverting the reflected sound waves to a voltage. The piezoelectricfilm of the detector 210 is provided to a low noise amplifier (notshown). A lock-in amplifier (not shown) coordinates the signalsgenerated by the pulse generator (not shown) and received by thereceiver. Processor 206 analyzes the different signals and can use thisinformation to develop the topology map.

In other embodiments, the sound detector 210 may be of the same typeused in the Meta-Pulse II or Meta-Pulse III Metrology Tool, sold by theRudolph Technologies, Inc. of Flanders, N.J. However, as shown inpresent FIGS. 2-3, the sound detector 210 is incorporated into thechamber 200 of the immersion photolithography apparatus for in-situtopography measurement.

The processor 206 is configured to calculate a local thickness of thephotoresist layer 150 at a plurality of locations 601-609 within thefield with sufficient precision to determine a local topography withinthe field, the calculation based on a round trip time of the sound. Theprocessor 206 commands the sound source 208 to generate the sound andreceives signals representing the received sound level from the detector210. The processor compares the time stamp of the sound emission to thestamp of the detection of the sound by the sound detector 210, andanalyzes the sound data to determine the local distance to the topsurface of the photoresist 150 at a plurality of locations within eachfield 601-609 (i.e., determine the topography). Although nine locations601-609 are indicated within the field, the thickness may be measured atany desired number of locations. The number of measured locations ineach field may be defined by considering the cycle time and desireddegree of accuracy. The more locations used, the more accurate theresult will be, but an increased number of focus and energy adjustmentswill result in a longer process time. The numbers of fields and dies aredifferent in different productions, so the process time accounts forthese factors.

The sound waves 213 are directed at the photoresist layer 150 on thefront side of the wafer 110. The receiver 210 detects the arrival ofreflected sound waves 215 from the wafer 110. By analyzing thetransmitted sound waves 213 and reflected sound waves 215, the localdistance from the transducers 208, 210 to the photoresist 150 can bedetermined at a plurality of locations 601-609 within each field.

In general, the distance between the transducer 208 and the photoresist150 can be determined from the total time (T1-T2), (i.e. the timeinterval between the transmission of an sound wave 213 by the transducer208 and the reception of the reflected wave 215 by the receiver 210) andby the speed of the sound waves through the medium (e.g., UPW). Thecalculation is simplified if the transducer 208 and receiver 210 areboth at the same height, but this is not a requirement, and one ofordinary skill can readily adapt the calculation for equipment in whichthe transducer 208 and receiver are at different heights. Therelationship can be expressed by the formula x=V(T1-T2)/2. In thisequation, V equals the velocity of the sound waves 213, 215 in themedium (e.g., speed of sound in water). The total travel time of thesound waves 213, 215 in turn is given by T1-T2, where T1 is the launchtime by the transducer 208 and T2 is the reception time by the receiver210.

As shown in FIGS. 3, 5 and 6, sound waves 213 are generated by thetransducer 208 continuously during leveling, at various locations601-609 within each field 600. These incident sound waves 213 are of aknown frequency, duration and amplitude. The reflected sound waves 215are detected by the receiver 210 after the round trip time period T1-T2.The frequency, duration and amplitude of the reflected waves 215 canalso be analyzed and compared to established data. Thus, the topographywithin each field 600 is determined by the processor 206.

Processor 206 determines a respective best focal length for lens 204corresponding to each respective location 601-609 within the field.Since the relative distance between the lens and the transducer 208 isknown, the focal length can be determined by adding the lens-transducerdistance to the measured distances collected during leveling.

In some embodiments, the transducer 208 generates and transmitsultrasonic sound waves, to reduce the ambient sound level of theequipment. In some embodiments, the transducer 208 may be configured togenerate sound waves at multiple frequencies, in turn. By performingseveral signal intensity measurements using the various frequencies, thesystem may provide even further accuracy in the detection of thetopography.

Processor 206 also determines a respective energy level for laser 102corresponding to each respective location 601-609 within the field. Theenergy levels are determined based on the focal length and empiricaldata.

The photolithography tool has a lens 204 for focusing light on the field600 of the substrate 110. The processor 206 is configured to generate afocus parameter for focusing the lens 204, based on the determinedtopography, and to provide a signal 207 for controlling the lens basedon the focus parameter.

The processor 206 is also configured to generate an energy levelparameter for the light source 102, based on the determined topography,and to provide a signal 209 for controlling an energy level of the lightsource based on the energy level parameter.

The tool further comprises a water supply (FIG. 4) for supplying water220 between the lens 204 and the substrate 110.

FIG. 4 shows additional details of one embodiment of an immersionlithography tool 400. The tool 400 has an ultra pure water (UPW) source402, a lens water supply (LWS) 404, a gap water supply 406, a lens waterarea 408 between the lens 102 and the substrate 110, a water surfaceextraction (WSE) region 410, an external air knife extraction 412, asource 414 of extreme clean humidified air, an air knife 416, an airknife extraction 418, a micro sieve 420, and a single phase extractionregion 422. This is only an example, and immersion lithography toolswith other configurations may be used.

FIG. 7 is a flow chart of one example of a method for using the SONARrelated photoresist thickness information to control a photolithographyprocess. Although the example of FIG. 7 describes the use of the methodfor measuring topography of a photoresist over a back end of line (BEOL)IMD layer, the method is not limited to BEOL layers. For example CopperCMP can create non-uniform topography; and IMD CMP and tungsten CMP canalso induce dishing and erosion because of POLY distribution on thedevice. The SONAR method described herein may also be applied to measurethe topography of any of these layers. Advantageously, the user of theseSONAR techniques is not limited by optical properties of the materialfor which the topography is measured.

At step 700, a semiconductor substrate 110 is provided, and the frontend of line (FEOL) processing is performed. This includes formation ofthe active device layers.

At step 702, after completion of the front end of line (FEOL)processing, an IMD layer 130 is formed over the substrate 110.

At step 704, the IMD layer 130 is patterned to form trenches. Then thetrenches are filled with conductive material (e.g., copper).

At step 706, CMP is performed to planarize the substrate.

At step 708, a photoresist layer 150 is applied over the planarized IMDlayer 130.

At step 710, the substrate 110 has been placed in the immersionphotolithography tool, and the immersion medium (e.g., UPW) 220 isflowed through the chamber, fully filling the space between the lens 204and the photoresist 150.

Steps 712 to 716 are performed in situ, without removing the substrate110 from the chamber of the immersion lithography tool. At step 712, asound 213 is applied to the substrate 110, such that a sound wave 215 isreflected by a top surface of the layer 150 of material.

At step 714, the sound wave 215 is detected using a sensor 210.

At step 716, a topography of the top surface is determined based on thedetected sound wave. The determining step is performed in situ in theimmersion photolithography tool. The topography is determined withsufficient precision to measure intra-field variations in surfaceheight. The determined topography can then be used to control animmersion lithography process.

At step 718, a loop including steps 720-724 is performed for each field.This loop may optionally be repeated plural times for each die. Forexample, a die may be patterned with two or more fields in pluralexposures, and the loop of steps 720-724 may be repeated to adjust thefocus and energy level separately for each respectively field in thedie.

At step 720, the focus of the immersion photolithography tool isadjusted based on the determined topography. The focus is adjusted to bewithin a window where both photoresist tapering and photoresist scum areavoided. The adjusting step is performed individually for scanning eachrespective field of the substrate,

At step 722, before exposing the photoresist, the energy level of a beamof the immersion photolithography tool is adjusted based on thedetermined topography. The energy is adjusted so that the after developinspection critical dimension of the patterned lines will be within theacceptable range (between minimum and maximum widths). The energy levelof the beam may be adjusted individually for exposing each respectivefield of the substrate.

At step 724, the photoresist is exposed after adjusting the focus.

In some embodiments, a method comprises: (a) providing a semiconductorsubstrate having at least one layer of a material over the substrate;(b) applying sound to the substrate, such that a sound wave is reflectedby a top surface of the layer of material; (c) detecting the sound waveusing a sensor; (d) determining a topography of the top surface based onthe detected sound wave; and (e) using the determined topography tocontrol an immersion lithography process.

In some embodiments, a method comprises: (a) providing a semiconductorsubstrate having an inter-metal dielectric layer with a plurality ofmetal patterns, and a layer of photoresist over the inter-metaldielectric layer; (b) applying sound to the substrate, such that a soundwave is reflected by a top surface of the layer of photoresist; (c)using a sensor to detect the sound wave; (d) determining a topography ofthe top surface based on the detected sound wave, with sufficientprecision to measure intra-field variations in surface height; (e)adjusting a focus of a photolithography tool and an energy level of abeam of the photolithography tool, based on the determined topography;and (f) exposing the photoresist after adjusting the focus and energylevel.

In some embodiments, an apparatus comprises an immersionphotolithography tool having a chamber configured for selectivelyexposing a portion of a photoresist layer above a substrate. The toolhas a light source for exposing a field of the photoresist layer. Asound source is positioned to emit a sound into the chamber, such thatthe sound is reflected from a top surface of the photoresist layer. Asound detector is positioned to receive the reflected sound. A processoris configured to calculate a local thickness of the photoresist layer ata plurality of locations within the field with sufficient precision todetermine a local topography within the field, the calculation based ona round trip time of the sound.

The methods and system described herein may be at least partiallyembodied in the form of computer-implemented processes and apparatus forpracticing those processes. The disclosed methods may also be at leastpartially embodied in the form of tangible, non-transient machinereadable storage media encoded with computer program code. The media mayinclude, for example, RAMs, ROMs, CD-ROMs, DVD-ROMs, BD-ROMs, hard diskdrives, flash memories, or any other non-transient machine-readablestorage medium, wherein, when the computer program code is loaded intoand executed by a computer, the computer becomes an apparatus forpracticing the method. The methods may also be at least partiallyembodied in the form of a computer into which computer program code isloaded and/or executed, such that, when the computer program code isloaded into and executed by a computer, the computer becomes anapparatus for practicing the methods. When implemented on ageneral-purpose processor, the computer program code segments configurethe processor to create specific logic circuits. The methods mayalternatively be at least partially embodied in a digital signalprocessor formed of application specific integrated circuits forperforming the methods.

Although the subject matter has been described in terms of exemplaryembodiments, it is not limited thereto. Rather, the appended claimsshould be construed broadly, to include other variants and embodiments,which may be made by those skilled in the art.

1. A method comprising: (a) providing a semiconductor substrate havingat least one layer of a material over the substrate; (b) applying soundto the substrate, such that a sound wave is reflected by a top surfaceof the layer of material; (c) detecting the sound wave using a sensor;(d) determining a topography of the top surface based on the detectedsound wave; and (e) using the determined topography to control animmersion lithography process.
 2. The method of claim 1, wherein thematerial is a photoresist, and step (e) comprises: adjusting a focus ofan immersion photolithography tool based on the determined topography;and exposing the photoresist after adjusting the focus.
 3. The method ofclaim 2, wherein the topography is determined with sufficient precisionto measure intra-field variations in surface height, and the adjustingstep is performed individually for scanning each respective field of thesubstrate,
 4. The method of claim 2, further comprising: before exposingthe photoresist, adjusting an energy level of a beam of the immersionphotolithography tool based on the determined topography.
 5. The methodof claim 4, wherein at least one of the focus and energy level of thebeam is adjusted individually at each of a plurality of locations withineach field for exposing each respective field of the substrate,
 6. Themethod of claim 2, wherein the determining step is performed in situ inthe immersion photolithography tool.
 7. The method of claim 6, whereinthe immersion photolithography tool has a lens that provides lightduring the exposing, the method further comprising: flowing liquidthrough a space between the top surface of the photoresist and the lens.8. The method of claim 7, wherein the liquid is water.
 9. The method ofclaim 2, wherein the substrate has an inter-metal dielectric layer belowthe layer of photoresist, and the inter-metal dielectric layer has aplurality of copper patterns.
 10. The method of claim 2, wherein steps(b), (c), (d), and (e) are performed in situ in a chamber withoutremoving the substrate from the chamber.
 11. A method comprising: (a)providing a semiconductor substrate having an inter-metal dielectriclayer with a plurality of metal patterns, and a layer of photoresistover the inter-metal dielectric layer; (b) applying sound to thesubstrate, such that a sound wave is reflected by a top surface of thelayer of photoresist; (c) using a sensor to detect the sound wave; and(d) determining a topography of the top surface based on the detectedsound wave, with sufficient precision to measure intra-field variationsin surface height; (e) adjusting a focus of a photolithography tool andan energy level of a beam of the photolithography tool, based on thedetermined topography; and (f) exposing the photoresist after adjustingthe focus and energy level.
 12. The method of claim 11, wherein theexposing comprises an immersion exposure process, and the determiningstep is performed in situ in the photolithography tool.
 13. The methodof claim 12, wherein the photolithography tool has a lens that provideslight during the immersion exposure process, the method furthercomprising: flowing water through a space between the top surface of thephotoresist and the lens.
 14. The method of claim 11, wherein the focusand energy level of the beam are adjusted individually for exposing eachrespective field of the substrate.
 15. The method of claim 11, whereinsteps (b), (c), (d), (e) ad (f) are performed in situ in a chamberwithout removing the substrate from the chamber.
 16. Apparatuscomprising: an immersion photolithography tool having a chamberconfigured for selectively exposing a portion of a photoresist layerabove a substrate, the tool having a light source for exposing a fieldof the photoresist layer; a sound source positioned to emit a sound intothe chamber, such that the sound is reflected from a top surface of thephotoresist layer; a sound detector positioned to receive the reflectedsound; and a processor configured to calculate a local thickness of thephotoresist layer at a plurality of locations within the field withsufficient precision to determine a local topography within the field,the calculation based on a round trip time of the sound.
 17. Theapparatus of claim 16, wherein the photolithography tool furthercomprises a lens, and the processor is configured to generate a focusparameter for focusing the lens, based on the determined topography, andto provide a signal for controlling the lens based on the focusparameter.
 18. The apparatus of claim 16, wherein the processor isconfigured to generate an energy level parameter for the light source,based on the determined topography, and to provide a signal forcontrolling an energy level of the light source based on the energylevel parameter.
 19. The apparatus of claim 16, wherein the tool furthercomprises: a lens for focusing light on the field; and a water supplyfor supplying water between the lens and the substrate.
 20. Theapparatus of claim 16, wherein the sound source and sound detector areboth positioned for operating within the chamber for in situ measurementof the local thickness within the same chamber in which the photoresistis exposed.